The crude petroleum oil, often referred to as charge, entering a petroleum refinery contains a number of impurities harmful to the efficient operation of the refinery and detrimental to the quality of the final petroleum product. Salts, such as primarily magnesium chloride, sodium chloride and calcium chloride, are present and generally range between 3 and 200 pounds per thousand barrels of crude. These salts are unstable at elevated temperatures. If allowed to remain with the petroleum charge throughout the various stages of the refinery operation the salts will dissociate and the chloride ion will hydrolyze to form hydrochloric acid. HCl, as well as organic acids which are present to varying degrees in the petroleum crude, contribute to corrosion in the main fractionator unit and other regions of the refinery system where temperatures are elevated, and where water condenses.
In addition to sodium, magnesium and calcium salts, other metal salts such as potassium, nickel, vanadium, copper, iron and zinc may be found in various concentrations. These metals contribute to heat exchanger fouling, furnace coking, catalyst poisoning and end product degradation.
Crude oil desalting is a common emulsion breaking method where the emulsion is first intentionally formed. Water is added in an amount of approximately between 5% and 10% by volume of crude. The added water is intimately mixed with the crude oil to contact the impurities therein, thereby transferring these impurities into the water phase of the emulsion. The emulsion is usually resolved with the assistance of emulsion breaking chemicals, which are characteristically surfactants, and by the known method of providing an electrical field to polarize the water droplets. Once the emulsion is broken, the water and petroleum media form distinct phases. The water phase is separated from the petroleum phase and subsequently removed from the desalter. The petroleum phase is directed further downstream for processing through the refinery operation.
Some of the impurities and water attempted to be removed by this method remain with the petroleum charge and ultimately result in the corrosion and fouling problems previously described. Various concepts which have attempted to resolve these continuing problems are described hereinbelow.